The present invention relates to solar cells and more particularly to solar cells having two heterojunctions.
Semiconductor solar cells are devices that convert solar radiation energy into electrical power. This conversion of energy occurs as a result of what is well known in the solar cell field as the "photovolatic effect". Two processes are involved in the "photovolatic effect". The first process is the generation in the active semiconductor material of electrons and holes as a result of the active materials absorption of solar radiation. Secondly, the electrons and holes are separated at some electrical inhomogeneity in the solar cell device, and it is this separation which results in the generation of an electrical current. A typical electrical inhomogeneity in a solar cell device is a P-N junction.
Solar cells having a single heterojunction are well known to those in the art. Such a solar cell would typically have a region of a semiconductor material which is substantially transparent to solar radiation through which radiation first enters the solar cell. In contact with this transparent region is the solar radiation absorbing region of the device which is of one conductivity type, and spaced from the transparent region but in intimate contact with the absorbing region is a bottom region of the same material as the absorbing region but of opposite conductivity. Single heterojunction solar cells can be more efficient than silicon solar cells, but solar radiation not absorbed in the absorbing region of a single heterojunction solar cell may be absorbed in the bottom region which is of the same material as the absorbing region. Thus, there is no opportunity for this unabsorbed solar radiation to be directed back toward the absorbing region for possible absorption. We have now found that a solar cell having a double heterojunction structure can have a bottom region which will allow unabsorbed solar radiation, from the absorbing region, travel to some radiation reflector for a second pass to the absorbing region for possible absorption. Increasing solar radiation absorption in the active region of a solar cell device will increase the efficiency of the device. Any increase in solar cell efficiency is most desirable.